The reaction of ClO with CH3OO forms CH3OCl3, which is thought to be an important species in ozone destruction over the Artic and Antarctic polar regions. Principal processes for CH3OCl loss include abstraction of an H atom by OH or Cl, where the C .H2OCl radical is formed. This radical can dissociate to Cl + CH2O products via beta scission (elimination) of a Cl atom with formation of a strong carbonyl pi bond; or if stable, C .H2OCl will react with O-2 to form a peroxy radical. Structures and the thermochemical properties (enthalpy, DeltaH(f298)degrees entropy S(298), and heat capacity) of the C .H2OCl radical, a transition state for its dissociation to CH2O + Cl (TSCH2O-Cl) and a formaldehyde-Cl atom adduct (ACH(2)O similar to Cl), are estimated by ah initio and density functional calculations. Geometries are optimized and frequencies are estimated using MP2/6-31G(d,p), B3LYP/6-31G(d,p), or MP2/6-31G(d) level calculations. Single point calculations for estimation of energy are performed with B3LYP/6-311 +G(3df,2p) and QC1SDT/6-31G(d,p) and with composite methods of CBS-Q and G3/MP2. Density functional calculations do not predict the existence of a stable C .H2OCl radical; only a lower energy, loosely bound adduct and final products CH2O + Cl. HF and MP2 calculations optimize to a C .H2OCl radical structure with an O-Cl bond (MP2) of 1.72 Angstrom. DeltaH(f298)degrees values on this MP2 structure are calculated using four different working (isodesmic) reactions. Standard enthalpy based on CBS-Q//MP2/6-31G(d,p) energies with isodesmic reaction analysis on this MP2 structure results in DeltaH(f298)degrees of 32.39 +/- 2.21 kcal/mol; this is in excellent agreement with the high level calculations of Espinosa-Garcia (32.0 +/- 3.5). MP2 calculations also predict an early transition state for C .H2OCl dissociation with an O-Cl bond of 1.78 Angstrom. Analysis of the MP2/6-31G transition state structure, using either CBS-Q or G3/MP2, yields an enthalpy some 4.4 kcal/mol lower than the stable radical; i.e., no barrier to C .H2OCl dissociation is found. Products CH2O + Cl and a loosely bound adduct of ACH(2)O similar to Cl have much lower energies at -1.86 +/- 2.21 and 0.26 +/- 2.21 kcal/mol, respectively. We conclude from analysis of the enthalpy values of the three structures (adduct, TST, and products) that the HF- or MP2-optimized C .H2OCl structure does not exist as a stable radical and that density functional calculations provide more realistic insight into the chemistry of this species. Abstraction of H atoms from CH3OCl by active radicals such as OH, Cl, O, H,..., etc., result in formation of CH2O plus a Cl atom.